This invention relates to an anti-reflection coating that is excellent in adhesion to an optical part made of synthetic resin. Reflection reducing effects are generally achieved by laminating plural layers of materials having different refractive indices.
There are many prior art reference referring to an anti-reflection coating which provides an intermediate layer made of silicon dioxide or silicon monoxide between a substrate and an anti-reflection coating (for instance Japanese Patent Application KOKAI Nos. 101901/1980, 130701/1985, 130702/1985, 130703/1985, 130704/1985 and 131501/1985).
One reason for providing such an intermediate layer made of silicon dioxide is to overcome the defect that a surface of an optical part made of synthetic resin is vulnerable to mechanical scratching and solvents such as chemicals so as to improve the surface hardness and durability.
The silicon dioxide layer has nearly an equal refractive index to that of the optical part made of synthetic resin, so the layer does not play a role in the reflection reducing effect but works as an intermediate layer that improves the adhesion of the optical part with the reflection reducing layer. Further the intermediate layer requires thickness of more than some specified degree and the required thickness results in the deterioration of the efficiency of reflection reducing effects and creates a difficulty in maintaining the surface profile.
While, a silicon monoxide layer is utilized in a similar way as the silicon dioxide because of its good adhesion with an optical part made of synthetic resin. The silicon monoxide layer, although, provides the problem that the reflectivity properties are unstable.
From above reasons, materials for reflecting reducing coatings that have reflection reducing effects in themselves, excellent adhesivity with optical parts made of synthetic resin and stability against age-based changes are desired.
Japanese Patent Application KOKAI No. 101901/1980 discloses that an anti-reflection layer is coated on a glass layer (about 2.mu..about.3.mu. in thickness) of silicon monoxide (SiO) or silicon dioxide (SiO.sub.2) formed on a synthetic resin substrate. The glass layer needs some specified thickness, which results in difficulty in controlling the thickness in the case of the glass layer formed by, for example, a dipping method and difficulty in maintaining the surface profile irregularity of optical parts and the layer causes the lack in the stability of reflectivity.
Japanese Patent Application KOKAI No. 130701/1985, 130702/1985, 130703/1985, 130704/1985, and 131501/1985 disclose an anti-reflection coating consisting of a silicon monoxide layer and silicon dioxide layer on a synthetic resin substrate. But the anti-reflection coating causes lack in the stability of reflectivity because the refractive index of the silicon monoxide layer changes with the passage of time.